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For example, Parrots have a much longer life expectancy than dogs. One theory is that the heart can only beat so many times, but parrots have a faster heart rate than dogs. So this theory doesn't work, right? I've been thinking about this question for years... I've asked, but never got a coherent answer. Hopefully, some day I'll get an answer.

I'm no biologist, so take this with a grain of salt. I think (maybe I've read this in a book or just did some thinking on my own, I'm not sure), that different species evolve to have different life expectancies in order to increase the likelihood of the descendants to survive.

A too short life and thus short timespan of being able to reproduce decreases the chance to reproduce, but on the other hand makes "immune" against some long term hazards, like e.g. diseases that take longer to incubate than the single animal lives.

Long life and long timespan for reproducing gives a bigger chance to reproduce, but also older animals will possible reproduce over and over again. This could have a dampening effect on new mutations in the gene pool. Which is a good thing for mutations that have negative effect, but also slows down the overall rate of adaption to new or changed selection pressures.

So a shorter timespan for reproduction will allow faster adaption to these new/changes selection pressures but also yields the chance that mutations with negative effects spread faster.

And the combination of a long life and short ability to reproduce has the effect that parents will be able to care/support their descendants even after the time they can reproduce.

As said, take this with a grain of salt, but I think that these are "just" different strategies that evolved to balance positive and negative effects and give a good chance for reproduction and reproduction of the descendants. After all, reproducing in terms of evolution seems bit of pointless if the descendants aren't then also able to reproduce.

I like your answer very much, and I forgot to mention that I've read about this theory. So field mice don't need to live so long because they produce so much offspring in such a short amount of time. Evolution... It makes perfect sense, however there's still a piece of the puzzle missing. Have biologist figured out what is it exactly that makes one particular species live longer than the other? Is it because they have better organs? This is what I'm trying to figure out.

I don't think there is something like "better quality organs". It is just that cells get damaged or destroyed over time and every time the DNA gets copied in the lifetime of an animal (or human for that matter), there is a chance of not copying it exactly correctly. I bet there is more, but you have different sources of degradation of the cells and DNA over time.

Some species could now evolve to counteract these effects by doing better replacement of damaged cells or better quality copies of their DNA. But nothing is for free. This costs energy and this energy could maybe better be used for other activities. For example some insects are very short living, like just a few days, but they spawn a lot of descendants in this short time. They could use this effort for other things, like increasing the life time, but then they either would have to decrease their rate of reproduction or "come up" with some solution for better energy usage or more energy input at all. Elephants for example have a very long life, but not that many descendants. I don't think an animal with the size and life expectancy of an elephant would be possible with the reproduction rate of those insects. Its just not possible in terms of energy requirements for the whole thing.

I quoted "come up" because its a figuratively speaking, since they don't actively consciously work towards solutions for these kind of problems.

So yeah, now short-living species could have a longer life, but that costs energy and so they would have to cut back on other fronts. Also please keep in mind, that I don't want to promote one "solution" over another, they are just different developments that are working for different environments.

This is fascinating stuff... Don't you agree? So hypothetically, If a long-living species like the elephant also had many offspring throughout its long life, it would counterintuitively be to it's detriment; since all these offspring would be competing for the same scarce food, and the majority of them would end up dying anyway; and that would be an inefficient strategy because lots of energy is going to waste in the process. So we can conclude by saying 'life is always seeking balance'. What do you think?

Is is fascinating! But I don't like "life is always seeking balance", because this implies that conscious effort to produce exactly this balance, which "life" (more evolution here) doesn't have. I, as a computer scientist, see evolution as the biological equivalent of a gradient descent in machine learning. It unconsciously optimizes the chance to produce offspring that itself will produce offspring. Each generation produces random offspring by recombination and mutation of existing gene material. The chance of this happening and the offspring also reproducing depends heavily on the environment and other life in it (e.g. predators). Changing this environment without the need to optimize for both environment (no gene flow between the groups in different environments) is the way new distinct species evolve. So no offense here, but I personally don't like "life is always seeking a balance" its more like "life is unconsciously optimizing probability" :-)